1. Field of the Invention
The present invention relates to an inflator suitable for an air bag system for an automobile.
2. Description of the Related Art
As an inflator for an inflating type safety system of an automobile, in order to optimally protect a passenger in accordance with a position of a seat in a vehicle such as a driver side, a passenger side and the like, various inflators are generally used. As such inflators, there have been known ones where such a pressurized gas as argon, helium or the like is used as an inflating means for an air bag.
In such an inflator, since an outflow of a pressurized gas is started by rupture of a rupturable plate and an air bag is inflated and developed finally, it is important to improve the rupturing ability of the rupturable plate for enhancing an activation reliability of the inflator. Further, a structure of the inflator is required to be simplified as much as possible in order to meet demands for size and weight reduction. Besides, there is a demand for simplification of an assembling process or the like.
In Japanese Patent Publication No. 2002-172995, there is disclosed an invention relating to a stored gas inflator. In FIG. 2 of the Japanese Publication a main chamber and a small chamber are formed, a communication hole and a small hole are formed in a separation wall disposed between these chambers, and rupturable plates (first and second burst shims) are mounted to a gas ejecting hole formed in the small chamber and the communication hole. An initiator 30 is provided in a pressurized atmosphere inside the small chamber, and in the description of the Japanese Publication, it is described that the rupturable plate can be ruptured even by an igniter with a small output. However, in this inflator, there are the following problems.
In this inflator, even the small chamber is maintained in a pressurized atmosphere. In paragraph of the description of the Japanese Publication, there is a statement that “P2 is approximately equal to or slightly smaller than (P1-Pm).” Here, P2 represents a rupturing pressure for rupturing the rupturable plate, P1 represents a rupturing pressure for rupturing the rupturable plate, and Pm represents a charging pressure of a gas charged in the small chamber and the main chamber. Considering the content of this disclosure, when the pressure in the small chamber becomes higher than that in the main chamber due to activation of the initiator 30, there occurs no problem when the two rupturable plates are simultaneously ruptured. However, in case that the rupturable plate has been ruptured first, since the pressure in the small chamber escapes to the main chamber, a pressure rising in the entire of the small chamber and the main chamber becomes small, so that the rupturable plate is not ruptured and an air bag can not be normally inflated. Further, the rupturable plate is not provided in the extension of the advancing direction of the impact wave at the time of activation of the initiator, which obviously is less reliable in view of rupturing the rupturable plate.
FIG. 3 of the same publication discloses a structure in which an igniter is disposed such that its distal end is directed to the rupturable plate. However, since a pressurized medium exists in the main chamber and the small chamber separately, two rupturable plates are required, which makes the structure complicated.
In U.S. patent Publication No. 2002/0093182, an inflator which ruptures a rupturable plate 9 by projecting a piston 23 is disclosed, as shown in FIGS. 1 to 3. Paragraph
and FIG. 4 of the US Patent Publication clearly explain that a hole is formed in a portion of the periphery of a member accommodating an igniter, an end portion of a member is entwisted therein and the piston is disposed inside. However, with such a projectile mechanism of the piston, the number of parts is increased and maintenance of size accuracy is complicated because of many small parts, and there is still room for improvement in these points.